The present invention relates to a leak test method and apparatus which are used to check various containers or vessels for leaks.
In the manufacture of products or parts required to be free of leaks, it is general practice in the prior art to inspect them in succession on production lines and compare the inspection data with preset reference values to determine if they are leak-free or not. With the conventional method, products under test for leaks (hereinafter referred to as works), such as vessels or containers, are tested successively on production lines by introducing thereinto compressed gas and making a check to see if the compressed gas leaks out thereof or if the amount of leakage is smaller than a reference value. A leak testing apparatus that has been used in the past is a differential pressure type leak tester that detects leaks of the compressed gas from the work based on variations in the pressure difference between a master tank (hereinafter referred to as a master) and the work.
Referring now to FIGS. 1 and 2, the differential pressure type leak tester will be described in brief. FIG. 1 is a piping diagram of a prior art example and FIG. 2 a graph showing how the abovementioned pressure difference changes with the lapse of time.
In FIG. 1, a pressurized gas source 10 is piped via a pressure regulation valve 12 and a three-way electromagnetic valve 14 to electromagnetic valves 16W and 16M leading to a work 22W and a master 22M, respectively. Connected between the work 22W and the master 22M is a differential pressure sensor 18. The three-way electromagnetic valve 14 normally vents the conduits on both its sodes to the atmospheric pressure, but in response to a drive voltage, conducts both the conduits to each other.
The actual leak test starts with opening the valves 12, 14, 16W, and 16M to introduce pressurized gas into the work 22W and the master 22M from the pressurized gas source 10, followed by regulating the regulation valve 12 to set a pressure gauge 13 at a desired testing pressure. Next, the electromagnetic valves 16W and 16M are closed and after a certain elapsed time the pressure difference between the work 22W and the master 22M is measured by the differential pressure sensor 18. If the work 22W leaks, the pressure on the work side becomes gradually lower than the pressure on the master side. The detected pressure difference is compared with a threshold value L.sub.th in a decision part 19. When the detected pressure difference is smaller than the threshold value L.sub.th, it is decided that the work 22W is leak-free or that the leak is negligibly small, and when the pressure difference is larger than the threshold value L.sub.th, the work 22W is decided to be leaky. In this case, if the work 22W does not leak, the pressure difference ought to be zero. In practice, however, the pressure difference frequently develops even if the work 22W does not leak. Such a situation might be the case wherein when the temperature of the work 22W heated on the production line is still higher than the temperature of the master 22M (room temperature, for instance), the measurement of the pressure difference is started and thereafter the pressure in the work 22W drops as its temperature is gradually reduced toward that of the master 22M as a result of thermal radiation. Even if no leaks are detected, the pressure difference usually varies due to temporal changes in the temperature difference between the work 22W and the master 22M. A description will be given below, with reference to FIG. 2, of how the pressure difference varies during measurements.
In FIG. 2, the curve 2A indicates the pressure difference when the work 22W does not leak and the curve 2B the pressure difference when the work 22W leaks. As shown, the pressure difference develops at and after time t.sub.s when the electromagnetic valves 16W and 16M depicted in FIG. 1 are closed, and thereafter the pressure difference varies unstably until time t.sub.a. This is primarily due to a shock resulting from the closing of the electromagnetic valves 16W and 16M. Then, the pressure difference undergoes substantially linear variation during the time interval from t.sub.a to t.sub.b. The reason for this is, for example, that the temperature of the pressurized gas introduced into the work 22W as mentioned above is gradually lowered. And the pressure difference varies in a smooth curve from time t.sub.b to t.sub.c, because the cooling rate of the pressurized gas reduces as its temperature approaches room temperature.
After time t.sub.c, the pressure difference does not vary when the work is free of leaks, but in the case of a leaky work, the pressure difference further undergoes linear variation. This period will hereinafter be referred to as a "stabilization period." Since the gas temperature in either of the work and the master is considered to be equal to room temperature during this period, pressure difference variation per unit time is in proportion to the amount of leakage (cm.sup.3 /sec). The amount of leakage that is intended to be detected is appreciably small, and it can be regarded as substantially constant from time t.sub.s to the stabilization period after time t.sub.c. Through utilization of this phenomenon it is possible to decide that the work is leak-free or leaky, depending upon whether the detected pressure difference variation per unit time after time t.sub.c is close to zero or not.
However, the conventional method is time-consuming since the measurement cannot be started until after time t.sub.c. A differential pressure type leak tester that has been proposed as a solution to this problem is disclosed in Japanese Patent Application Laid-Open Gazette No. 4-506262. With this leak tester, the pressure difference variation per unit time is premeasured using a leak-free, non-defective work in the time interval t.sub.a to t.sub.b depicted in FIG. 2 during which the pressure difference varies linearly after time t.sub.a when it ceases from sharp variations. The abovementioned period from time t.sub.a to t.sub.b will hereinafter be referred to as a "measurement period."
In the actual leak test, pressure difference variation per unit time, .DELTA.p/.DELTA.t, is detected in the measuring period t.sub.a to t.sub.b, and is compared with the pressure difference variation premeasured using the leak-free, non-defective work. It is possible to decide that the product under test is leak-free or leaky, depending upon whether the pressure difference values compared are nearly equal or not. This enables the measurement to start prior to time t.sub.c and hence permits reduction of the measurement time.
This prior art method is effective in reducing the measurement time but requires the preparation of a non-defective work. To solve this problem there has been proposed such a method as described below.
To begin with, two pressure differences p.sub.1 and p.sub.2 are measured, using a sample work, at a predetermined time interval .DELTA.t (two seconds, for instance) in the measurement period from time t.sub.a to t.sub.b shown in FIG. 3, and pressure difference variation per unit time, .delta.p.sub.1 =(p.sub.2 -p.sub.1)/.DELTA.t=.delta.p.sub.1 /.delta.t, is calculated from the two pressure differences p.sub.1 and p.sub.2. This is followed by calculating pressure difference variation per unit time, .delta.p.sub.2, from two pressure differences p.sub.3 and p.sub.4 similarly measured at the predetermined time interval .DELTA.t in the stabilization period after time t.sub.c. The pressure difference variations per unit time, .delta.p.sub.1 and .delta.p.sub.2, are calculated as follows: EQU .delta.p.sub.1 =(p.sub.2 -p.sub.1)/.DELTA.t=.DELTA.p.sub.1 /.DELTA.t EQU .delta.p.sub.2 =(p.sub.4 -p.sub.3)/.DELTA.t=.DELTA.p.sub.2 /.DELTA.t
The pressure difference variation .delta.p.sub.2 in the stable period can be regarded as a variation attributable to leaks of the work. The pressure difference variation .delta.p.sub.1 in the measurement period can be regarded as the sum of the abovementioned variation .delta.p.sub.2 and the amount of variation which occurs even in the absence of leaks (which variation will hereinafter referred to as the "amount of drift" or simply as "drift"). Thus, the drift p.sub.d can be obtained by subtracting the pressure difference variation .delta.p.sub.2 from .delta.p.sub.1. In this way, the drift, p.sub.d =.delta.p.sub.1 -.delta.p.sub.2, is precalculated.
In the actual leak test, the pressure difference variation per unit time is calculated first on the work under test during the time interval between t.sub.1 to t.sub.2, and the abovementioned drift is subtracted from the pressure difference variation to obtain a correction result. Finally, the presence or absence of leaks is decided, depending on whether the correction result falls within a predetermined range of values.
This method permits correction using one sample work (without distinction of leakage), and hence does not involve the preparation of leak-free, non-defective work. Further, the pressure difference variation needs only to be measured on the sample work until the stabilization period, and the measurements can be made on other works at earlier timing to determine if they are leak-free or leaky.
With the prior art, however, it is necessary to measure the pressure differences p.sub.1 and p.sub.2 in the measurement period and the pressure differences p.sub.3 and p.sub.4 in the stabilization period by a single differential pressure sensor as depicted in FIG. 3. In general, there is a limit to the number of digits of each value that a single differential pressure sensor can measure with accuracy. The minimum resolution of a differential pressure sensor capable of measuring large pressure differences is more coarse than the minimum resolution of a differential pressure sensor for measuring small pressure differences. For this reason, a small pressure difference (for example, 0.1 mmHg) cannot be detected with high accuracy by a differential pressure sensor for large pressure differences (for example, 100 mmHg). Hence, the prior art encounters difficulty in calculating drift with high accuracy.
On the other hand, the proposal made in Japanese Patent Application Laid-Open Gazette No. 4-506262 mentioned above involves the preparation of a leak-free, non-defective work.